Accumulation of pathogenic helper CD4 T cells and failure of regulatory T cell (Treg)-mediated suppression are now considered as central to understanding the role of adaptive immunity in autoimmune diseases, including inflammatory bowel disease (IBD). Multiple mechanisms by which Tregs control intestinal inflammation have been suggested but additional mechanisms may exist particularly to maintain homeostasis in the intestinal microenvironment. Our analysis of Wnt ligand expression in distinct lymphoid cell populations revealed that Tregs express the Wnt antagonist Dickkopf1 (Dkk1) significantly higher than other major Wnt ligands, suggesting a potentially important role of Tregs secreting this potent Wnt antagonist. To pursue mechanistic studies we generated a recombinant form of Dkk1 produced in mammalian cells. The findings were notable in that the rDkk1 demonstrated the marked inhibitory effect on naive CD4 T proliferation and stimulation of Treg proliferation. Our preliminary analysis with Tregs from doubleridge mice (Dkk-1 d/d) that express very low level of Dkk-1 showed that indeed these Tregs failed to protect host from colitis by failure to suppress CD4 T cell proliferation. This study utilizing the standard T-cell mediated Inflammatory Bowel Disease (IBD) model allowed us to uncover important properties of Treg that warrant an extensive mechanistic characterization of their function. More importantly, further analysis of CD4 T cell differentiation results showed that rDkk1 inhibited Th1 cell differentiation and markedly elevates IL-10. All of these findings have lead to the hypothesis that Treg-derived Dkk1 is a potent regulator to prevent intestinal inflammation that is critical in IBD. To test thi hypothesis in Aim 1, the mechanism by which Dkk1 inhibits cell proliferation of naive CD4 T cells will be determined in vitro. Naive CD4 T cells lacking the canonical receptor will be utilize to determine whether canonical signaling is required. These cells will also be tested for function in vivo. Finally, we investigate the mechanism to inhibit Th1 polarization and the elevation of IL-10 by Dkk-1. In Aim 2, we will generate Dkk1 deficient Treg utilizing Dkk1 floxed mice. These Dkk1 deficient Treg will be evaluated functionally in the in vivo IBD model. In vitro, the stimulatory effect of Dkk-1 on Tregs will be studied regarding signal transduction pathways that may lead to Treg proliferation. Treg will also be generated that lack the Wnt canonical receptor Lrp5 for assessing the effects of Dkk1 on signaling pathways activating proliferation. We expect these studies to reveal fundamentally important but currently unrecognized properties of Tregs that are essential to understanding autoimmune disease in the intestine but are also generally relevant to autoimmunity. This will likely lead to increased efforts to target Wnt signaling for clinical applications, particularly in Crohn's disease and other autoimmune diseases.